This invention is concerned with locating and tracing concealed elongated conductive objects, such as pipes or cables, and is more particularly concerned with improved locating and tracing of a first object when a second object is adjacent to the first.
In the prior art, there are two general techniques of locating buried metallic objects. A passive technique employs a gradiometer or the like as a magnetic locator for detecting the presence of ferrous metal objects, such as iron and steel pipes, iron markers, manhole covers, well casings, etc. An active technique uses a transmitter to induce alternating currents in non-ferrous metal pipes, power cables, or communication cables, for example, and a receiver to sense magnetic fields associated with the currents.
The model MAC-51B Magnetic and Cable Locator manufactured by the assignee of the present invention is designed for selective active or passive use. When apparatus of this type is employed to locate and trace a cable (or non-ferrous pipe), for example, a transmitter may be disposed on the ground at a position close to the location (or suspected location) of a portion of the cable so as to induce an alternating current therein that may be traced by moving a receiver back and forth over the ground. When there are no interfering objects close to the cable being traced, this system works admirably, producing a distinct single null in the output signal of the receiver when the receiver is located directly over the cable and is oriented so as to sense a vertical component of a circumferential magnetic field associated with the current in the cable. When, however, another cable (or pipe) is present adjacent to the first cable, e.g., within a few feet of the first cable and extending in the same general direction, the single null output signal characteristic of the receiver becomes distorted, and tracing of the desired cable may become difficult.